The Rat Asialoglycoprotein Receptor Binds the Amino-Terminal Domain of Thyroglobulin

Abstract

We have previously reported that the rat hepatic lectin-1 (RHL-1) subunit of rat asialoglycoprotein receptor (ASGPr), the endocytic receptor found on the basolateral surface of hepatocytes, was expressed in rat thyroid tissue and localized on the apical surface of polarized rat thyroid FRT cells. Here we show that PC Cl3 cells, a differentiated rat thyroid cell line, bound thyroglobulin (Tg) via ASGPr. In fact, both the bacterial recombinant carbohydrate recognition domain of RHL-1 (rCRDRHL-1) and the anti-rCRDRHL-1 antibody markedly inhibited 125I-Tg binding to the cell surface of PC Cl3 cells. Ligand blot assays with deglycosylated Tg show that the rCRDRHL-1 was able to interact with Tg even after remotion of sugars. The region of Tg involved in the binding to RHL-1 was investigated by ligand blot assays with biotinylated rCRDRHL-1 on thermolysin-digested native and desialated rat thyroglobulin. It is shown that the rCRDRHL-1 specifically recognized a thyroglobulin fragment with an apparent Mr of 68,000, corresponding to the amino-terminal part of the molecule. To our knowledge, this is the first report that attributes to the amino-terminal portion of Tg molecule, containing its earliest and major hormonogenic site, the function of binding to a cell surface receptor of the thyroid. Moreover, we show that oligosaccharides are not the only molecular signals for binding to RHL-1, but amino acidic determinants could also play a role.